31 Don't stop exercising! An investigation of the effect of different levels of physical activity was conducted on identical male twins in Finland. For each twin pair, al- though they had maintained the same activity level for most of their lives, one twin had significantly reduced his or her activity over the last few years due to work or family pressures. For each pair of twins, body fat per- centage, endurance levels, and insulin sensitivity were measured. For the less active twin, the results showed greater body fat, worse endurance, and levels of insulin sensitivity displaying early signs of metabolic disease.23 (a) What type of design is being used in this investiga- tion? Give the explanatory and response variables. (b) Is this an experiment or an observational study? Why? (c) The article reports that the measurements were car-